1. Field of the Invention
The present invention relates to a capacitance-type physical quantity sensor provided with a sensor circuit including at least one charge amplifier for converting a capacitance signal into a voltage signal.
2. Description of Related Art
There is known, as such a capacitance type physical quantity sensor, a vibration type gyro sensor (refer to IEEE JOURNAL OF SOLID-STATE CIRCUITS VOL. 37 NO. 12 “Single-Chip Surface Micromachined Integrated Gyroscope With 50°/h Allan Deviation”, for example). In the gyro sensor of this type, detection signals outputted from a vibrator as complementary capacitance signals are converted into voltage signals by charge amplifiers, and then supplied to a differential amplifier. The differential between the voltage signals amplified by the differential amplifier undergoes synchronous detection, zero-point adjustment, temperature-characteristic compensation, etc. to make a sensor output signal.
The charge amplifier of such a gyro sensor is constituted by an operational amplifier which needs a feedback resistor of high resistance. FIG. 3 is a circuit diagram of a conventional charge amplifier usable for such a gyro sensor.
As shown in this figure, the charge amplifier is constituted by an operational amplifier J1 applied with a detection signal (capacitance signal) at its inverting input terminal and applied with a reference voltage at its non-inverting input terminal. A capacitor J2 and a MOS resistor J3 as a feedback resistor are connected in parallel between the inverting input terminal and output terminal of the operational amplifier J1.
The MOS resistor J3 is constituted by a series of MOS transistors J4 and J5, a capacitor J6 connected at one terminal thereof to a node of the MOS transistors J4, J5, a MOS transistor J7 whose gate is connected to the source thereof and to the gates of the MOS transistors J4, J5, and a constant current source J8 connected between the source of the MOS transistor J7 and the ground. The other terminal of the capacitor J6 and the drain of the MOS transistor J7 are applied with the reference voltage.
By controlling the current drawn by the constant current source J8, on which the resistances of the MOS transistors J4, J5 are dependent, it is possible that the MOS resistor J3 has a desired high resistance.
However, a refresh time (a time period required for the electric charge accumulated in the capacitor J2 to be discharged) of the charge amplifier having the above described structure is considerably long, because the time constant of the CR circuit formed by the capacitor J2 and the MOS resistor J3 is large due to the high resistance of the MOS resistor J3. This poses a problem that the time necessary for carrying out inspection of a wafer on which a sensor circuit including charge amplifiers having the structure as described above is formed becomes long, because the charge amplifiers have to be refreshed to check their offset values in this inspection.